Ceramic resistor contacted barrier-free with noble metal



H. SEITER April 25, 1967 CERAMIC RESISTOR CONTACTED BARRIER-FREE WITHNOBLE METAL 2 Sheets-Sheet 1 Original Filed Jan. 29, 1963 228 1% mm TI:

am nm E o t l o Tm K; 3 mg #9 m April 25, 1967 H. SEITER 3,316,518

CERAMIC RESISTOR CONTACTED BARRIERFREE WITH NOBLE METAL Original FiledJan. 29, 1965 2 Sheets-Sheet 2 Fig.2

Fig.3

3,316,518 Patented Apr. 25, 1967 3,316,518 CERAMIC RESISTOR CONTACTEDBARRIER-FREE WITH NOBLE METAL Hartmut Seiter, Munich, Germany, assignorto Siemens & Halske Aktiengesellschaft Berlin and Munich, a corporationof Germany. I Continuation of application Ser. No. 254,757, Jan. 29,1963. This application Aug. 2, 1966, Ser. No. 569,762 Claims priority,application (germany, Feb. 2, 1962, 7 85 3 Claims. (Cl. ass-45 This is acontinuation of application Ser. No. 254,757, filed J an. 29, 1963, nowabandoned.

The invention disclosed herein relates to a resistor with hightemperature coefiicient and is particularly concerned with thebarrier-free contacting of a ceramic resistor made of dopedferroelectric crystallite particles which are sintered together, suchresistor having a high temperature coefficient of its total resistancevalue.

It was found, in connection with the manufacture of such resistors, thatthey often exhibit an undesired voltage dependence of the resistancevalue. In order to avoid such voltage dependence or to reduce it to aharmless extent, it was proposed, according to Patent No. 3,027,529,dated Mar. 27, 1962, to provide the contacts, substantially free ofbarrier layers, upon the material of the resistor, and it was furtherproposed to use for the contacting a common metal, especially aluminumor zinc.

The relationship of the varistor effect, that is, the voltage dependenceof the resistance value and the kind of the contact metal, wereinvestigated in the course of further experiments, whereby it was foundthat the barrier freedom of the contacts depends greatly on theinfluence of the contact metal upon the ceramic material.

The provision of a substantially barrier-free contact is possible whenthe contacting metal is adapted to reduce the four valent metal oxide ofthe ferroelectric basic metal, which is contained in the ceramicmaterial; the term reduce or reducing, being used here in the sense of areduction of the oxygen content without disturbance of the grid, suchreduction being linked with an increase of the surface conductivity.This applies to the common metals noted in the above mentioned patent,since. the formation enthalpy of their oxide is at least equal to oronly slightly smaller than the difference of the formation enthalpies ofthe trivalent or four-valent oxide of the four-valent metal in theferroelectric basic material.

According to the above noted patent, the formation of a contact, free ofa barrier layer, therefore requires, in connection with a ceramicresistor, provision, upon the ceramic semiconductor material, of acontact metal which is adapted to reduce the four-valent metal oxide ofthe ferroelectric basic material contained in the ceramic body, wherebythe formation enthalpy of an oxide of the contact metal is at leastapproximately equal to or only slightly lower than the difference of theformation enthalpies of the trivalent or four-valent oxides of thefour-valent metal in the ferroelectric basic material.

It is assumed that this reduction leads to an over-doping in the surfaceregion, thus preventing formation of a barrier layer.

The experimental results shall now be discussed with reference to anexample of an embodiment illustrated in the accompanying drawings.

FIG. 1 explains the formation enthalpy AH of the metal oxides withrespect to the varistor effect;

FIG. 2 shows an example of an embodiment of a resistor according to theinvention; and

FIG. 3 is a graph showing the resistance course of a resistor body whichis contacted barrier-free.

In connection with the illustrated example, the formation enthalpy ofthe oxide of the contact metal was used as a measure for the reductionfunction thereof. A reduction from four-valent to trivalent oxide is bythis contacting metal possible, when this formation enthalpy is greater(more negative) than that of the transition of trivalent to four-valentoxide of the four-valent metal in the ferroelectric basic material.

As indicated before, FIG. 1 shows the formation enthalpy AH of the metaloxide (respectively referred to 1 atom oxygen) plotted against thevaristor effect. The resistance condition at the measuring voltage 0.2and 5 volts (R 0.2 v./R 5 v.) was thereby used as a reference value forthe varistor effect. An alteration of the resistance or of the dopingdoes not change anything with respect to the basic differences of thevaristor effect at different metals. Instead of the formation enthalpyAH, there should be, more accurately, entered the free formationenthalpy which results from AH with consideration of the entropy.However, this correction of the formation enthalpy amounts at the mostto 3 kcaL/mole and therefore is unimportant.

The formation enthalpy of the transition from Ti 0 to TiO amounts,according to the equation 2TiO =Ti O +l/20 to AH=62.5 kcaL/mole It willbe seen from FIG. 1 that a considerable reduction of the varistor effecttakes place in case of a contact metal having a negative oxide formingenthalpy greater than about +55 kcaL/mole, and that a substantiallybarrier-free contact can be obtained with a metal having a negativeoxide forming enthalpy greater than about +60 kcL/mole. Accordingly,practically no varistor effect occurs in the thermodynamic existancerange of the Ti 0 becoming, however, very pronounced in the region inwhich no reduction of the Ti0 is possible. It will also be seen fromFIG. 1 that the varistor effect lies with silver approximately one tenspower higher than in the case of comparable metal as, for example,palladium.

It can be assumed, based upon these results, that the ferroelectricmaterial, especially a perovskite formed of divalent and four-valentmetals, primarily the barium titanate, is incident to the contactingreduced in place on the surface, thereby becoming strongly conductive,so that no barrier effect can anymore appear.

In order to avoid being bound to the use of common contacting metals,the ceramic body is according to the invention reduced on the surfacealong the regions of areas thereof which are to be contacted. This maybe advantageously done, for example, by electrolytically separatinghydrogen at the surface of the ceramic body. The use of the glow effect,that is, of producing a glow discharge at the surface, is often likewiseof advantage. A noble metal, especially a metal which can be solderedwell, for example, copper, and which would have a strong varistor effectwithout the surface reduction, is after the reduction precipitated whilethe reducing condition is maintained. A barrier-free contact is therebylikewise formed, and the ceramic body is thus contacted in electricallysatisfactorily and solderable manner.

FIG. 2 shows an embodiment of a resistor according to the invention.Numeral 1 indicates the rod-shaped resistor body which is sintered offerroelectric crystallites, such body particularly consisting of aceramic semiconductor built up on a barium titanate basis. The endsurfaces of the resistor body are provided with metal coatings indicatedat 4 and 5, which are, for example, electrolytically separated andconsist, for example, of copper having an oxide-forming enthalpyamounting to about -38 kcaL/mole, which is, accordingly, lower (morepositive) than the formation enthalpy of the transition from four-valentto trivalent titanium oxide. The contact is nevertheless barrier-freesince the end surfaces 2 and 3 are prior to providing the noble metalthereon, reduced,

for example, by conducting thcreover a hot reducing gas.

The metal layers vor coatings may also be produced, for

example, by vaporization instead of electrolytically.

In FIG. 3, the resistance is entered along the ordinate and the voltagealong the abscissa, both on a logarithmic scale. The curve 1 shows theresistance course of a resistor body which is contacted barrier-free.There is practically no varistor effect. The curve 2 represents theresistance course of a resistor body which is contacted with copperwithout previous reduction of the contact area. The formation enthalpyof the copper oxide amounts to about -38 kcal./mole. The curve 2 shows astrong voltage dependence of the resistor, which is due to theconditions described before.

, Changes may be made within the scope and spirit of the appended claimswhich define what is believed to be new and desired to have protected byLetters Patent.

I claim: I i

1. A ceramic semiconductor resistor comprising a body having oppositeend surfaces and consisting of sintered together doped ferroelectriccrystallite particles with perovskite-structure from two-valent andfour-valent metals with high positive temperature coefficient ofresistance and with a substantially negligible voltage dependence of theentire resistance, the ceramic semiconductor body being provided at theopposite end surfaces thereof with barrier-free contact areas byconducting a hot reducing gas thereover, a pair of barrier-free metalcontact layers each disposed on a respective opposite end surface ofsaid body, said contact layers being a noble metal which is ineffectiveto reduce the four-valent metal oxide of the ferroelectric materialcontained in said body, said device having a characteristic such thatthe negative oxide-forming enthalpy of said noble metal is smaller thanthe formation enthalpy of the transisition from Ti O to TiO 2. Aresistor according to claim 1, wherein the ferroelectric material ofsaid body contains titanium, and wherein the negative oxide-formingenthalpy of said contact metal is at least smaller than 60 kcaL/mole.

3. A resistor according to claim 1, wherein the negative oxide-formingenthalpy of said contact metal issmaller than kcal./mole.

References Cited by the Examiner UNITED STATES PATENTS RICHARD M. wooo,Primary Examiner. W. D. BROOKS, Assistant Examiner.

1. A CERAMIC SEMICONDUCTOR RESISTOR COMPRISING A BODY HAVING OPPOSITEEND SURFACES AND CONSISTING OF SINTERED TOGETHER DOPED FERROELECTRICCRYSTALLITE PARTICLES WITH PEROVSKITE-STRUCTURE FROM TWO-VALENT ANDFOUR-VALENT METALS WITH HIGH POSITIVE TEMPERATURE COEFFICIENT TORESISTANCE AND WITH A SUBSTANTIALLY NEGLIGIBLE VOLTAGE DEPENDENCE OF THEENTIRE RESISTANCE, THE CERAMIC SEMICONDUCTOR BODY BEING PROVIDED AT THEOPPOSITE END SURFACES THEREOF WITH BARRIER-FREE CONTACT AREAS BYCONDUCTING A HOT REDUCING GAS THEREOVER, A PAIR OF BARRIER-FREE METALCONTACT LAYERS EACH DISPOSED ON A RESPECTIVE OPPOSITE END SURFACE OFSAID BODY, SAID CONTACT LAYERS BEING A NOBLE MEATL WHICH IS INEFFECTIVETO REDUCE THE FOUR-VALENT METAL OXIDE OF THE FERROELECTRIC MATERIALCONTAINED IN SAID BODY, SAID DEVICE HAVING A CHARACTERISTIC SUCH THATTHE NEGATIVE OXIDE-FORMING ENTHALPY OF SAID NOBLE METAL IS SMALLER THANTHE FORMATION ENTHAPLY OF THE TRANSISITION FROM TI2O3 TO TIO2.